In recent years, various information other than speech such as images and data etc. has become the subject of transmission in mobile communication. This means that requirements with regards to high reliability and high-speed transmission have increased. However, in the event that high-speed transmission is carried out in mobile communication, the influence of wave delays due to multi-paths cannot be ignored, and cause deterioration in transmission characteristics due to frequency-selective fading.
Multicarrier communication typified by OFDM (Orthogonal Frequency Division Multiplexing) methods are noted as one technology for counteracting frequency-selective fading. Multicarrier communication is technology for carrying out high-speed transmission by carrying out data transmission using a plurality of subcarriers for which transmission speed is suppressed to an extent where frequency-selective fading does not occur. In particular, with OFDM methods, the frequencies of the plurality of subcarriers on which data is arranged are mutually orthogonal. The frequency utilization efficiency is therefore the highest within multicarrier communication and can be implemented with comparatively simple hardware. Because of this, OFDM methods are noted as a candidate communication method for adoption in fourth generation mobile communication and are currently being subjected to various examinations.
For example, in non-patent document 1, the application of transmission power control typically employed in CDMA (Code Division Multiple Access) method communication in OFDM methods is being examined. Non-patent document 1 discloses technology for individually controlling transmission power of a plurality of subcarriers.
The operation for individually controlling transmission power of the subcarriers is described giving a specific example with reference to FIG. 1A and FIG. 1B.
FIG. 1A is a view showing power of each subcarrier before carrying out transmission power control. A description is given of the case for controlling transmission power of six mutually orthogonal subcarriers of frequency f1 to f6.
The power of each subcarrier is taken to be, for example, 10 for subcarrier of frequency f1, 8 for subcarrier of frequency f2, 6 for subcarrier of frequency f3, 5 for subcarrier of frequency f4, 3 for subcarrier of frequency f5, and 2 for subcarrier of frequency f6. It is further taken that transmission data is modulated using the QPSK (Quadrature Phase Shift Keying) method.
Under the conditions described above, at a receiving apparatus, when target transmission power per bit for obtaining the desired SIR (Signal to Interference Ratio) is taken to be 3, target transmission power for one symbol (i.e. two bits) is 6 because transmitted data is modulated using the QPSK method. This target transmission power can be determined at either transmission apparatus or receiving apparatus.
In the case of deciding using transmission apparatus, received power at receiving apparatus for subcarriers for all of the frequencies f1 to f6 is fed-back to the transmission apparatus, and target transmission power is determined based on these received powers taking into consideration transmission path loss.
On the other hand, in the case of deciding at receiving apparatus, target transmission force occurring at a transmission apparatus is determined from the received power occurring at the receiving apparatus, and transmission power control commands (hereinafter abbreviated to “TPC”) relating to the respective subcarriers are fed-back to the transmission apparatus in such a manner that the transmission power each subcarrier becomes the target transmission power.
As shown in FIG. 1B, the transmission apparatus exerts control in such a manner that transmission power of each subcarrier becomes a target transmission force of 6 according to target transmission power determined by the subject apparatus or TPC commands for each subcarrier fed-back from the receiving apparatus.
Namely, the power of subcarrier of frequency f1 is reduced by 4, the power of subcarrier of frequency f2 is reduced by 2, the power of subcarrier of frequency f3 remains the same, the power of subcarrier of frequency f4 is increased by 1, the power of subcarrier of frequency f5 is increased by 3, and the power of frequency f6 is increased by 4.
It is then possible to suppress the generation of peak power specific to multicarrier communication by carrying out transmission power control, and it is possible to achieve the desired SIR at the receiving apparatus.
(Non-patent Document 1) Yoshiki, Sanpei, Morinaga: “Performance of a Multilevel Transmit Power Control Scheme for the OFDM Subcarrier Adaptive Modulation System”, Technical Report of IEICE. SSE2000-71, RCS2000-60 (2000-07), pp. 63-68.